Since the first observation of carbon nanotubes, numerous papers have reported studies on the yield of well-graphitized nanotubes, their diameter and wall thickness (single or multiple), growth mechanisms, alignment, electron emission properties, nanodevices, theoretical predictions, and potential applications. Selective positioning and growth of carbon nanotubes is necessary for future integration with conventional microelectronics as well as the development of novel devices. However, limited progress has been reported in the controlled placement of nanotubes. Alignment of the carbon nanotubes is particularly important to enable both fundamental studies and applications, such as cold-cathode flat panel displays, chargeable batteries, and vacuum microelectronics.
Specifically, vertical alignment has been an important goal due to its technological importance for applications such as scanning probe microscopy and field emission flat panel displays. Attempts to manipulate nanotubes for these applications have been made by post-growth methods such as cutting a polymer resin-nanotube composite, or drawing a nanotube-ethanol suspension through a ceramic filter. Because these techniques are difficult and labor intensive, in situ aligning of nanotubes during growth using techniques such as the nanopores of porous alumina membranes and laser etched nanotracts have been attempted.
There has been little success in obtaining alignment of carbon nanotubes on large areas until the report by Li et al., “Large-Scale Synthesis of Aligned Carbon Nanotubes,” Science, 274:1701-1703 (1996) (“Li”). Li discusses the growth of aligned carbon nanotubes on mesoporous silica containing iron nanoparticles via thermal decomposition of acetylene gas in nitrogen gas at temperatures above 700° C. In this method, the substrate is prepared by a sol-gel process from tetraethoxysilane hydrolysis in iron nitrate aqueous solution. The gel is then calcined 10 hours at 450° C. at 10−2 Torr. A silica network with relatively uniform pores is obtained with iron oxide nanoparticles embedded in the pores. The iron oxide nanoparticles are then reduced at 550° C. in 180 Torr of flowing (9% H2/N2 (110 cm3/min) for 5 hours to obtain iron nanoparticles. Thereafter, nanotubes are grown in a gas environment of a mixture of 9% acetylene in nitrogen at 700° C. Aligned nanotube growth is along the axial direction of the pores. Only the nanotubes which grow out of the vertical pores are aligned. Nanotubes which grow from the iron particles on the surface and in the dispersed, inclined pores are random and non-oriented. In this method, nanotube alignment is limited to the constraint of the vertically aligned pores. Further, the density and diameter of aligned carbon nanotubes is respectively limited in direct proportion to the amount and size of the iron nanoparticles and the diameter of the pores.
As disclosed in Li, a temperature of at least 700° C. is required to decompose acetylene and induce carbon nanotube growth. Unfortunately, this high temperature requirement limits substrate selection. For example, a glass substrate is unsuited for use in this method due to its low strain point temperature. A glass produced by Corning Incorporated (Corning, N.Y.) has the highest known flat panel display glass deformation or strain point temperature of 666° C. Typically, a commercially available flat panel display glass has a strain point temperature between 500° C. and 590° C. At 700° C., glass substrates deform and inhibit aligned carbon nanotube growth. Accordingly, any substrate suitable for use with this method must have a melting point or strain point temperature above 700° C.
Terrones et al., “Controlled Production of Aligned-Nanotube Bundles,” Nature, 388: 52-55 (1997) (“Terrones”) disclose a method for laser induced growth of nanotube bundles aligned on a substrate under high temperature conditions. A thin film of cobalt is deposited on a silica plate by laser ablation and thereafter etched with a single laser pulse to create linear nanotracks. 2-amino-4,6-dichloro-s-triazine is then disposed onto the silica plate in the presence of argon gas within a two stage oven. The first oven is heated to 1,000° C. and then allowed to cool to room temperature. The second oven is heated to and maintained at 950° C. Although carbon nanotubes grow along the edges of the eroded nanotracks, growth only occurs on the substrate bottom surface and in a non-vertical fashion. Carbon nanotubes do not grow on a similarly prepared substrate top surface which indicates nanotube growth according to this method is gravity dependent. Again, for the reasons discussed above, substrate selection for this method is limited to a substrate having either a strain point or melting point temperature above 1,000° C. Further, nanotube density is directly limited to the number of nanotracks etched into the substrate surface.
Thus, there remains a need for a method of forming aligned, vertically or otherwise, carbon nanotubes at temperatures below 700° C. Similarly, there remains a need for a substrate which has carbon nanotubes vertically aligned on the substrate surface. Further, there remains a need for a method of forming individual, free-standing carbon nanotubes, and a substrate with one or more individual, free-standing carbon nanotubes disposed on the substrate surface. The present invention is directed to overcoming these deficiencies in the art.